[Home](./index.md) > [@firebase/auth](./auth.md) > [Auth](./auth.auth.md) ## Auth interface Interface representing Firebase Auth service. Signature: ```typescript export interface Auth ``` ## Remarks See [Firebase Authentication](https://firebase.google.com/docs/auth/) for a full guide on how to use the Firebase Auth service. ## Properties | Property | Type | Description | | --- | --- | --- | | [config](./auth.auth.config.md) | [Config](./auth.config.md) | The [Config](./auth.config.md) used to initialize this instance. | | [currentUser](./auth.auth.currentuser.md) | [User](./auth.user.md) \| null | The currently signed-in user (or null). | | [emulatorConfig](./auth.auth.emulatorconfig.md) | EmulatorConfig \| null | The current emulator configuration (or null). | | [languageCode](./auth.auth.languagecode.md) | string \| null | The Auth instance's language code. | | [name](./auth.auth.name.md) | string | The name of the app associated with the Auth service instance. | | [settings](./auth.auth.settings.md) | [AuthSettings](./auth.authsettings.md) | The Auth instance's settings. | | [tenantId](./auth.auth.tenantid.md) | string \| null | The Auth instance's tenant ID. | ## Methods | Method | Description | | --- | --- | | [onAuthStateChanged(nextOrObserver, error, completed)](./auth.auth.onauthstatechanged.md) | Adds an observer for changes to the user's sign-in state. | | [onIdTokenChanged(nextOrObserver, error, completed)](./auth.auth.onidtokenchanged.md) | Adds an observer for changes to the signed-in user's ID token. | | [setPersistence(persistence)](./auth.auth.setpersistence.md) | Changes the type of persistence on the Auth instance. | | [signOut()](./auth.auth.signout.md) | Signs out the current user. | | [updateCurrentUser(user)](./auth.auth.updatecurrentuser.md) | Asynchronously sets the provided user as [Auth.currentUser](./auth.auth.currentuser.md) on the [Auth](./auth.auth.md) instance. | | [useDeviceLanguage()](./auth.auth.usedevicelanguage.md) | Sets the current language to the default device/browser preference. |